The transition between the liquid or rubbery state and the glassy state is not sharp, but a glass temperature can be defined in terms of measurements of volume or heat capacity. This temperature can be related to the relaxation regions in visco-elastic or dielectric behaviour. Theories of the glass transition relate this variously to molecular relaxations, to free volume and to excess enthalpy or entropy; these are not altogether mutually exclusive. Observed transition temperatures can be related qualitatively with molecular structure, but a quantitative treatment faces formidable difficulties. Properties of glassy polymers reflect the generally low cohesion and the frozen-in disorder and hole structure. Relaxation behaviour is observed in the vicinity of the glass temperature.